1. Field of the Invention
This invention relates to enhanced oil recovery processes, utilizing a novel surfactant mixture comprising a mixture of mono- and di-alkyl benzene polyethoxy alkyl sulfonates wherein each alkyl group has the same number of carbon atoms in both components of the mixture.
This invention also relates to novel fluid compositions useful in oil recovery processes, herein comprising the aqueous solution of the above surfactant mixture.
2. Description of the Prior Art
The crude oil which has accumulated in subterranean reservoirs is recovered or produced through one or more wells drilled into the reservoir. Initial production of the crude oil is accomplished by "primary recovery" techniques wherein only the natural forces present in the reservoir are utilized to produce the oil. However, upon depletion of these natural forces and the termination of primary recovery a large portion of the crude oil remains trapped within the reservoir. Also, many reservoirs lack sufficient natural forces to be produced by primary methods from the very beginning. Recognition of these facts has lead to the development and use of many enhanced oil recovery techniques. Most of these techniques involve injection of at least one fluid into the reservoir to produce an additional amount of the crude oil therefrom. Some of the more common methods are water flooding, steam flooding, miscible flooding, CO.sub.2 flooding, polymer flooding, surfactant flooding, caustic flooding, and in situ combustion.
Water flooding, which involves injection of water into the subterranean oil reservoir for the purpose of displacing the crude oil from the pore spaces of the reservoir rock toward the producing wells is the most economical and widely used of the enhanced oil recovery methods. Nevertheless, water does not displace oil with high efficiency because of the immiscibility between water and oil and because of the high interfacial tension between them.
Surfactant flooding involves the addition of one or more surface active agents, or surfactants, to the water flood for the purpose of minimizing the water flooding problems mentioned above. This has been an area of active interest in the art of enhanced oil recovery methods for many years. For example, in 1941, U.S. Pat. No. 2,233,381 disclosed the use of polyglycol ether as a surfactant which increases the capillary displacement efficiency of an aqueous flood. U.S. Pat. No. 3,302,713 discloses the use of petroleum sulfonates as effective surfactants in oil recovery operations. Other surfactants proposed for use in oil recovery processes include alkyl pyridinium salts, alkyl sulfonates, alkyl aryl sulfates, ethoxylated alkyl or alkyl aryl sulfates, alkyl sulfonates, alkyl aryl sulfonates, and quaternary ammonium salts.
While the above surfactants may be effective under ideal conditions, there are problems concerning the use of each in most petroleum reservoirs. Some of the most serious problems arise from the effects of the reservoir fluid salinity on the injected surfactant. The problems include the spontaneous formation of immobile emulsions and precipitation of the surfactant. Under such conditions, the art contains proposals to mix various chemically different types of surfactants, such as anionic and nonionic, to achieve a higher salinity tolerance for the mixture. However, even this is often not entirely satisfactory because as the mixture containing the dissimilar surfactants is driven through the formation, one of the surfactant components is commonly preferentially adsorbed to the mineral grains in the reservoir matrices, causing a change in the relative concentration of the surfactant components and resulting in a failure to maintain effective salinity tolerance.
The economic success of any oil recovery process will depend in large part on the efficiency and cost of the particular surfactant chemicals employed. It is therefore desirable to use a surfactant composition that can be synthesized at a relatively low cost and yet will recover oil with high efficiency.